1. Field of the Invention
The present invention relates to an automatic transmission for a vehicle, and more particularly, an improvement in the arrangement of the clutches incorporated in the automatic transmission for a vehicle.
2. Description of the Prior Art
An automatic transmission for a vehicle generally comprises an input rotational member, an output rotational member, a rotational speed change gear mechanism, and clutches and brakes for changing over a route of transmittance of torque between said input rotational member and said output rotational member through said rotational speed change gear mechanism in accordance with selective engagement and disengagement of the clutches and the brakes, so as selectively to provide a plurality of gear stages such as forward speed stages and a reverse stage. In the automatic transmission of this kind the torque can not generally be transmitted monotonously from one axial end thereof toward the other axial end thereof, but must be transmitted repeatedly forward and backward in the transmission mechanism in order to provide various speed stages at desired gear ratios. Therefore, the clutches and the brakes incorporated in the transmission mechanism are generally allowed to occupy only a limited annular space corresponding to a relatively small part of the radial dimension available in a generally circular cross section of the transmission mechanism, so as to allow other torque transmitting members to pass therethrough or therearound. Therefore, in order to have a required torque transmitting capacity the clutches and the brakes are generally constructed to have a number of annular friction elements piled up one over the other in the axial direction with the alternate ones thereof being respectively torque transmittingly carried by an annular input clutch member and an annular output clutch member telescopingly arranged relative to one another. Therefore, the clutches and the brakes generally require a relatively large axial dimension.
In view of such particular conditions it has been considered to arrange two clutches in a radially duplicate structure so that a first clutch having a first rotational cylinder member and a first rotational piston member received therein to construct a first cylinder-piston actuator for selectively axially compressing an axial pile of annular friction elements for the first clutch houses radially therein a second clutch having a second rotational piston member received in said first rotational piston member which also serves as a second rotational cylinder member for the second clutch so as thereby to construct a second cylinder-piston actuator positioned radially inside said first cylinder-piston actuator for selectively axially compressing an axial pile of friction elements for the second clutch, as described in U.S. Pat. No. 4,716,787.
In such a radially duplicate clutch structure said first rotational piston member operates as one of the input and output rotational members of the radially inside clutch, and therefore the first rotational piston member must be torque transmittingly connected with the first rotational cylinder member which also serves as one of the input and output rotational members of the radially inside clutch. Therefore, the first rotational cylinder member and the first rotational piston member are formed with axial splines at a radially inside surface portion thereof and at a radially outside surface portion thereof, respectively, and engaged with one another to transmit torque therebetween, while allowing the first rotational piston member to move relative to the first rotational cylinder member in axial directions.
In such a radially duplicate clutch structure, if the torque transmittance is switched over from the first or outside clutch to the second or inside clutch for switching over gear stages of the transmission, a pressure fluid which has been supplied in a pressure chamber defined between the first rotational cylinder member and the first rotational piston member will be exhausted while a fluid pressure will be newly introduced into a second pressure chamber defined between the first rotational piston member and the second rotational piston member. However, if the second clutch starts to transmit a substantial torque before the first rotational piston member retracts sufficiently to release the compression which has been applied to the axial pile of the annular friction elements for the first clutch, the torque load applied between the first rotational cylinder member and the first rotational piston member via said spline engagement therebetween will brake a smooth retracting movement of the first rotational piston member relative to the first rotational cylinder member. If this once occurs, as this braking torque increases more as the second clutch is more tightly engaged for full operation thereof, the first clutch will remain in a half engaged condition throughout the operating period of the second clutch and will suffer heavy wearing. In a more serious instance, if the supply of pressure fluid to the second pressure chamber is more early relative to the discharge of the pressure fluid from the first pressure chamber, the first clutch will remain in the engaged condition, so that the transmission will be erroneously shifted to another speed stage in which the first and the second clutches are both engaged. According to the design of the return spring (70) which is constructed to have such a relatively moderate spring force as to be readily overcome by the force of servo pressure supplied either to the first pressure chamber (65) or the second pressure chamber (67) to allow either the first rotational piston member (62) or the second rotational piston member (63) to move against the return spring so as to satisfactorily compress the axial pile of the corresponding annular friction elements, the full return of the first rotational piston member (62) relative to the first rotational cylinder member (54) will be guaranteed only when no substantial torque is applied between the first rotational cylinder member (54) and the first rotational piston member (62). In fact, in the transmission shown in the above-mentioned U.S. Pat. No. 4,716,787, the first clutch (C1) is engaged to provide 1st through 3rd speed stages, (4th speed in the gear train of FIG. 6) while the second clutch (C2) is engaged to provide the reverse stage with disengagement of the first clutch or is engaged together with the first clutch to provide the 3rd speed stage (4th speed in the gear train of FIG. 6). Therefore, it might occur that if the driver hurries to back the vehicle when the manual lever has been set at the "D" range position, the automatic transmission provides the 3rd speed stage (4th speed stage in this case) or, if the matter is not so serious, the friction elements of the 1st clutch suffer heavy wearing.